1. Field of the Invention
The present invention relates to a heat fixing device for fixing toner of a copying device and which is for use at a low total pressure of less than 20 kg. The term "total pressure" as used herein refers to the sum of the pressures over the entire length of a roller.
2. Description of the Prior Art
Conventional fixing device of the heat roll type generally in use, for example, those comprising a heat fixing roller and a pressure roller in pressing contact therewith under a pressure of 40 to 150 kg have the problem of creasing paper when fixing images to envelopes or the like having a multilayer structure. To overcome this problem, i.e., to make envelopes or the like usable as copy paper without creasing, some of such fixing devices are adjustable so that the pressure roller is pressed against the heat fixing roller at a low total pressure of less than 20 kg.
FIG. 4 is an enlarged fragmentary sectional view showing a conventional heat fixing roller lA for use in fixing devices of the heat roll type. The roller 1A comprises a Teflon resin layer 11a formed on the surface of a core 10a and having a thickness of tens of microns. Since the resin layer 11a has almost no elasticity, the roller has high hardness at the portion thereof to be brought into contact with toner T1. Accordingly, the fixing device for use at a low total pressure of less than 20 kg is operable without creasing the paper but exerts too low a pressure on copy paper 3a, such as a letterhead, having surface indentations or projections, with the result that a space L1 occurs between the surface of the heat fixing roller 1A and a portion of the toner T1 deposited on the indented portion 31a of the surface of copy paper 3a as shown in FIG. 4. The heat fixing roller 1A is therefore unable to fully apply its heat and pressure to the toner portion T1, consequently failing to properly fix the toner portion T1 to the copy paper 3a. If such improper fixing becomes pronounced, an offset will occur. This phenomenon can be precluded by decreasing the hardness of the heat fixing roller to provide an increased nip width, but this method entails the problem of impairing the proper passage of plain paper through the device.